repository: enable initialization of submodules

repository: enable initialization of submodules

Introduce 'repo_submodule_init()' which performs initialization of a
'struct repository' as a submodule of another 'struct repository'.

The resulting submodule 'struct repository' can be in one of three states:

  1. The submodule is initialized and has a worktree.

  2. The submodule is initialized but does not have a worktree.  This
     would occur when the submodule's gitdir is present in the
     superproject's 'gitdir/modules/' directory yet the submodule has not
     been checked out in superproject's worktree.

  3. The submodule remains uninitialized due to an error in the
     initialization process or there is no matching submodule at the
     provided path in the superproject.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>